High strength polyethylene fibers and their applications

ABSTRACT

High strength polyethylene fibers and their applications, for example, chopped fibers, ropes, nets, ballistic materials or items, protective gloves, fiber reinforced concrete products, helmets, and other products obtained therefrom, in which the fiber is characterized in that: it contains a high molecular weight polyethylene consisting essentially of a repeating unit of ethylene; it has an intrinsic viscosity number of 5 or larger and an average strength of 22 cN/dtex or higher; and the measurement of the fiber by differential scanning calorimetry (DSC) exhibits a temperature-increasing DCS curve having at least one endothermic peak over a temperature region of 140° C. to 148° C. (on the low temperature side) and at least one endothermic peak over a temperature region of 148° C. or higher (on the high temperature side) or the number of frictions until the fiber is broken in an abrasion test according to method B for measuring abrasion resistance in the Testing Methods for Spun Yarn (JIS L 1095) is 100,000 or larger.

TECHNICAL FIELD

[0001] The present invention relates to novel high strength polyethylenefibers and their applications. More particularly, it relates to highstrength polyethylene fibers which can be widely used in various fieldsfor industry, for example, as the chopped fiber fibers or staples toproduce non-woven fabrics or spun yarns; as the ropes or nets forindustrial or private use; as the materials for high performancetextiles such as ballistic materials or items, or protective gloves; oras the reinforcing fibers for composite materials such as fiberreinforced concrete products or helmets.

BACKGROUND ART

[0002] For high strength polyethylene fibers, there have been disclosed,for example, in JP-B 60-47922, high strength, high modulus fibersproduced by the “gel spinning method” using ultrahigh molecular weightpolyethylene as the base material. These high strength polyethylenefibers have already been widely used in various fields for industry, forexample, as the ropes or nets for industrial or private use; as highperformance textiles such as ballistic materials or items, or protectivegloves; or as the geo-textiles or working nets in the filed of civilengineering and architecture.

[0003] In recent years, these high strength polyethylene fibers havebeen required to have further improved performance, particularlydurability, for example, mechanical durability over a long period oradaptability under severe conditions in use. Even textiles such assportswears, or fishing lines have also been required to have durabilitywhen used for a long period. In addition, reinforcing sheets or strandsto provide earthquake resistance have been required to have durability,particularly flexural fatigue resistance or abrasion resistance, suchthat when wound around pillars or other parts they cause no occurrenceof fiber breaking at the corners.

[0004] The high strength polyethylene fibers have excellent tensilestrength and excellent Young's modulus indeed, but on the other hand,the structure of their highly-oriented molecular chains is responsiblefor the drawback that they have poor durability, particularly poorflexural fatigue resistance and poor abrasion resistance, for example,as compared with polyesters or nylons for ordinary garments. Suchdrawback has become some obstacle to the wide application of highstrength polyethylene fibers in various fields for industry.

[0005] Further, many attempts have been made to use high strengthpolyethylene fibers in the chemical processes, for example, applicationto non-woven fabrics such as chemical filters or battery cellseparators, because of their excellent resistance to chemicals, lightand weather or to apply high strength polyethylene fibers to reinforcingfibers for concrete or cement, because there has been a demand for fiberreinforced concrete products having high crack resistance and hightoughness, as well as excellent impact resistance and excellentlong-term durability, since accidents were caused by wall materialscoming off or falling from the surface of railroad tunnels or bridges.

[0006] However, when chopped fibers or staples are produced by cuttingthe conventional high strength polyethylene fibers, fibrillation of thefibers or their high surface hardness is responsible for the drawbackthat these fibers get stuck together by pressure to form a bundle offibers, lacking in dispersibility. Further, when they are used as thereinforcing fibers for concrete or cement, their dispersibility in thecement matrix becomes deteriorated by flexure or entanglement of thefibers. For this reason, various treatments have been needed, forexample, premixing with cement, hydrophilicity-providing treatment usingmetal oxides, or binding with resins.

DISCLOSURE OF INVENTION

[0007] To overcome such drawbacks, the orientation of the extendedpolyethylene molecular chains should be more relaxed, which method,however, causes a lowering of strength and Young's modulus and cannot,therefore, be employed. Further, polyethylene fibers have no stronginteraction between the molecular chains and easily cause fibrillationby repeated fatigue, which makes it very difficult to improve thedurability of these fibers.

[0008] Thus an objective of the present invention is to provide highstrength polyethylene fibers and their applications, which fibers haveabout the same or higher strength and Young's modulus than those of theconventional high strength polyethylene fibers, and further haveexcellent flexural fatigue resistance and excellent abrasion resistance,and hardly cause fibrillation, and still further have high surfacehardness.

[0009] That is, the present invention relates to high strengthpolyethylene fibers characterized in that: the fiber comprises a highmolecular weight polyethylene consisting essentially of a repeating unitof ethylene; it has an intrinsic viscosity number of 5 or larger and anaverage strength of 22 cN/-dtex or higher; and the measurement of thefiber by differential scanning calorimetry (DSC) exhibits atemperature-increasing DCS curve having at least one endothermic peakover a temperature region of 140° C. to 148° C. (on the low temperatureside) and at least one endothermic peak over a temperature region of148° C. or higher (on the high temperature side).

[0010] The present invention further relates to high strengthpolyethylene fibers characterized in that: the fiber comprises a highmolecular weight polyethylene consisting essentially of a repeating unitof ethylene; it has an intrinsic viscosity number of 5 or larger and anaverage strength of 22 cN/-dtex or higher; and the number of frictionsuntil the fiber is broken in an abrasion test according to method B formeasuring abrasion resistance in the Testing Methods for Spun Yarn (JISL 1095) is 100,000 or larger.

[0011] The present invention still further relates to chopped fibers,ropes, nets, ballistic materials or items, protective gloves, fiberreinforced concrete products, helmets, and other products obtained fromthe above high strength polyethylene fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows a temperature-increasing DSC curve obtained bydifferential scanning calorimetry (DSC) of the high strengthpolyethylene fiber of Example 1.

[0013]FIG. 2 shows a temperature-increasing DSC curve obtained bydifferential scanning calorimetry (DSC) of the high strengthpolyethylene fiber of Example 2.

[0014]FIG. 3 shows a temperature-increasing DSC curve obtained bydifferential scanning calorimetry (DSC) of the high strengthpolyethylene fiber of Example 3.

[0015]FIG. 4 shows a temperature-increasing DSC curve obtained bydifferential scanning calorimetry (DSC) of the high strengthpolyethylene fiber of Comparative Example 1.

[0016]FIG. 5 shows a temperature-increasing DSC curve obtained bydifferential scanning calorimetry (DSC) of the high strengthpolyethylene fiber of Comparative Example 2.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The high strength polyethylene fibers of the present inventionare composed of a high molecular weight polyethylene consistingessentially of a repeating unit of ethylene. As used herein, theexpression “high molecular weight polyethylene consisting essentially ofa repeating unit of ethylene” refers to a polyethylene, which may beregarded essentially as an ethylene homopolymer containing a repeatingunit of ethylene at a ratio of 99.5 mol % or higher, preferably 99.8 mol% or higher, and which has an intrinsic viscosity number of 5 or larger,preferably 8 or larger, and more preferably 10 or larger. For thepurpose of increasing the rate of polymerization, or the purpose ofimproving the creep and other characteristics of finally obtainedfibers, the introduction of branches into the polyethylene by theaddition of copolymerizable monomers such as α-olefins in very smallamounts is recommended; however, higher amounts of copolymerizablemonomers are not preferred for improving the durability of fibersbecause it is, for example, presumed that copolymerization withα-olefins prevents mutual sliding between the molecular chains in thecrystals, which makes it impossible to achieve the relaxation of stressfor continuously repeated deformation. If the base polymer has anintrinsic viscosity number of smaller than 5, it is difficult to exhibitthe mechanical characteristics of fibers, particularly tensile strength.On the other hand, there is no upper limit to the intrinsic viscositynumber; however, taking into consideration the stability andproductivity in the yarn making process, the durability of fibers, andother factors, it is preferred that the intrinsic viscosity number is 30or smaller. Intrinsic viscosity numbers larger than 30 may cause, forexample, the lowering of durability in some cases depending upon drawingconditions for spun yarns.

[0018] Thus the high strength polyethylene fibers of the presentinvention composed of a high molecular weight polyethylene consistingessentially of a repeating unit of ethylene come to have an intrinsicviscosity number of 5 or larger. As used herein, the intrinsic viscositynumber of fibers refers to a corresponding value obtained by viscositymeasurement in decalin at 135° C. and extrapolation of η_(sp)/c (whereη_(sp) is specific viscosity and c is concentration) toward zeroconcentration. In actual cases, viscosity measurement is carried out forsome concentrations, and a straight line is drawn on the plot ofspecific viscosity η_(sp) against concentration c by the method of leastsquares and extrapolated toward zero concentration to determine anintrinsic viscosity number.

[0019] Further, the high molecular weight polyethylene as the basepolymer is not particularly limited, so long as finally obtained fibersmeet the above intrinsic viscosity number. For improving the durabilityof fibers to its limit, the use of a base polymer having more narrowmolecular weight distribution is preferred. The use of a base polymerhaving a molecular weight distribution index (Mw/Mn) of 5 or lower,which polymer is obtained using polymerization catalysts such asmetallocene catalysts, is more preferred.

[0020] The high strength polyethylene fibers of the present inventionhave an average strength of 22 cN/dtex or higher. As used herein, theaverage strength refers to an average value of strength (cN/dtex)obtained by drawing of a strain-stress curve using a tensile testerunder the conditions: length of specimen, 200 mm (gap distance betweenchucks); elongation rate, 100%/-min.; atmospheric temperature, 20° C.;and relative humidity, 65%; and calculation from the stress at thebreaking point on the curve obtained (number of measurements, 10).

[0021] For the high strength polyethylene fibers of the presentinvention, their measurement by differential scanning calorimetry (DSC)exhibits a temperature-increasing DCS curve having at least oneendothermic peak over a temperature region of 140° C. to 148° C. (on thelow temperature side) and at least one endothermic peak over atemperature region of 148° C. or higher (on the high temperature side).The temperature-increasing DSC curve is obtained by using a specimen offibers, which have been cut in 5 mm or shorter length, keeping thespecimen in a completely free state under an atmosphere of an inert gas,and heating the specimen from room temperature up to 200° C. at aheating rate of 10° C./min. For the endothermic peaks, only employed arepeaks, of which peak temperatures can be read, and the increased DSCcurve obtained is corrected for the base line, followed by reading ofpeak temperatures and peak heights. As used herein, the base line refersto the part of a DSC curve in the temperature region where no transitionnor reaction occurs in the test specimen as defined in the TestingMethods for Transition Temperatures of Plastics (JIS K 7121). The peakheight refers to the distance measured vertically to the axis ofabscissa between an interposed base line and the peak crest. In theTesting Methods for Transition Temperatures of Plastics (JIS K 7121),the peak is defined as the part of a DSG curve where the curve leavesthe base line and then returns to the same base line. In the presentinvention, when the temperature-increasing DSC curve obtained isdifferentiated (ie., the first derivative curve is drawn) and the valueof derivative (ie., the distance measured vertically to the axis ofabscissa between the first derivative curve and the axis of abscissa;the value of derivative has a plus or minus sign, if the curve is aboveor below the axis of abscissa, respectively) changes its sign from plusto minus, such part of the curve is defined as a peak, and the part ofthe curve where the value of derivative changes from the monotonousincrease to the monotonous decrease while keeping its plus or minus signis defined as a shoulder. From this definition, for example, it followsthat the DSC curve shown in FIG. 2 has two peaks and the DSC curve shownin FIG. 4 has one peak and one shoulder.

[0022] By the way, JP-A 63-275708 discloses high strength polyethylenefibers obtained by a special technique using copolymerization witha-olefins, and describes that when these fibers are wound around analuminum pan to stand in a constrained condition under tension and thensubjected to measurement by differential scanning calorimetry (DSC), twoor more peaks arising from copolymerization, in addition to the mainpeak, are observed on the high temperature side. It is, however, wellknown that when high strength polyethylene fibers in such a constrainedcondition under tension are subjected to DSC measurement, it usuallycauses an increase of the melting point, or in some case, the occurrenceof two or more peaks arising from crystal transition or other factors.

[0023] In contrast, the high strength polyethylene fibers of the presentinvention are composed of a polyethylene which can be regardedessentially as an ethylene homopolymer, and the measurement bydifferential scanning calorimetry (DSC) in the present invention iscarried out by using a specimen of fibers, which have been cut in 5 mmor shorter length, and keeping the specimen in a completely free state.To the inventors' knowledge, no report has been made in the past abouthigh strength polyethylene fibers exhibiting, even in such case, two ormore endothermic peaks on the high temperature side. The reason for theoccurrence of two or more endothermic peaks on the high temperature sideeven in such completely free state seems to be the presence of ahigh-temperature melting-type crystal structure (hereinafter referred toas “HMC”) different from the ordinary polyethylene crystal (hereinafterreferred to as “EC”). As shown in Examples, favorable results areobtained when structure formation is achieved by more positive removalof solvents contained in the fiber surface. It can therefore be presumedthat HMC is preferably formed on the surface layer of fibers, which HMClayer has a function to keep the strength of fibers and is a factor inthe expression of extremely excellent flexural fatigue resistance andextremely excellent abrasion resistance. It is also presumed thatexcellent abrasion resistance prevents fibrillation and forms the fibersurface with high hardness.

[0024] JP-A 61-289111 discloses semi-stretched yarns obtained by thespinning method using two kinds of special solvents and describes thattheir DSC curves drawn by measurement in a “free state” has two or moreendothermic peaks. Although there is no other way but to guess what isthis “free state”, it is well known that two or more endothermic peaksmay often be observed even when fibers, which have not been cut short,are inserted in an aluminum pan for measurement because although it canbe said that these fibers are in a freer state than that in the ordinarymeasurement with fibers wound around a small piece of aluminum, thefibers in the pan are, in fact, partly fixed between the bottom and thecover of the pan or there occurs uneven distribution of stress over thespecimen. To avoid such influence in the measurement, a specimen shouldcarefully be cut in very short length as done by the present inventors.Even if the measurement disclosed in the above publication is the sameas that of the present invention, the temperature region of endothermicpeaks disclosed in the above publication is different from that of thepresent invention, it is presumed from the following reason that thestretched yarns disclosed therein have poor flexural fatigue resistanceand poor abrasion resistance. In the meantime, with the producing methoddisclosed in the above publication, ie., slow technique in which thefirst and the second solvents are substantially removed just afterspinning, it is quite difficult to provide the fiber surface with aclose structure.

[0025] As described above, for the high strength polyethylene fibers ofthe present invention, the temperature-increasing DSC curve thereof hasat least one endothermic peak over a temperature region of 140° C. to148° C. In particular, such peak is preferably the main peakcorresponding to the largest value of heat flow among two or moreendothermic peaks found in the temperature-increasing DSC curve. It ispresumed that the main peak reflects the ordinary structure (EC)occupying the major part of fibers, and if the peak temperature thereofis lower than 140° C., the fibers have insufficient heat resistance. Incontrast, if the peak temperature thereof is higher than 148° C., theordinary fiber structure becomes highly restricted, for example, anaggregate of completely-extended chains, lowering the durability offibers. The present inventors have been found that the durability offibers, particularly flexural fatigue resistance in this case, becomesoptimum when the main peak appears over a temperature region of 140° C.to 148° C.

[0026] For the high strength polyethylene fibers of the presentinvention, the temperature-increasing DSC curve thereof further has atleast one endothermic peak over a temperature region of 148° C. orhigher (on the high temperature side). It is presumed that thisendothermic peak on the high temperature side corresponds to the HMCstructure having great influence on durability, particularly abrasionresistance, of which formation mechanism will be described below; andfibers exhibiting no endothermic peak on the high temperature side haveextremely deteriorated abrasion resistance.

[0027] As described above, it is presumed that the maximum endothermicpeak on the high temperature side among two or more endothermic peaksfound in the temperature-increasing DSC curves of the high strengthpolyethylene fibers of the present invention is derived from the HMCstructure. Adjusting the height of this maximum endothermic peak on thehigh temperature side makes it possible to obtain high strengthpolyethylene fibers having optimum durability.

[0028] In general, the fatigue of molecular-oriented fibers, of whichtypical examples are high strength polyethylene fibers, from flexure orabrasion is mainly caused by the fibrillation of the fibers from thesurface layer. It is presumed that the high strength polyethylene fibersof the present invention have the surface layer of HMC with moreentangled molecular chains, which results in a structure hardly causingfibrillation; therefore, the closer surface structure renders the fibersexcellent flexural fatigue resistance and abrasion resistance,preventing the fibers from getting stuck together by pressure when theyare cut.

[0029] It is, however, important that the high strength polyethylenefibers of the present invention should have a particular ratio of HMCoccupying the whole crystal structure. As described above, it ispresumed that the maximum endothermic peak on the high temperature sideis derived from the fusion of EC and the maximum endothermic peak on thelow temperature side is derived from the fusion of HMC. The height ratioof these maximum endothermic peaks over the respective temperatureregions is usually in the range of 1.4:1.0 to 3.0:1.0, preferably1.5:1.0 to 2.9:1.0, and more preferably 1.6:1.0 to 2.8:1.0. If the ratiois lower than 1.4:1.0, ie., if the maximum endothermic peak on the hightemperature side is relatively higher, this means that the ratio of HMCforming the surface layer of fibers is higher, which lowers thedurability of fibers. This is probably because an excessive increase inthe surface hardness promotes deterioration such as buckling fatigue. Incontrast, if the ratio is higher than 3.0:1.0, ie., if the maximumendothermic peak on the high temperature side is relatively lower, theratio of HMC is lower, which makes no trouble for strength or Young'smodulus but also makes no improvement in durability, so that fiberscannot be prevented from getting stuck together by pressure when theyare cut, making is impossible to obtain chopped fibers having gooddispersibility.

[0030] Further, the surface HMC structure according to the presentinvention is very effective for the improvement of impact resistance. Toobtain high impact resistance, fibers are required to have high strengthand high degree of elongation in the deformation at high strain rate,what is called, toughness. The surface HMC structure according to thepresent invention has a function to improve both of thesecharacteristics. From the viewpoint of viscoelastic properties, polymermaterials may be considered a combination of elastic components andviscous components as explained by, what is called, the Takayanagimodel. In case of deformation at high strain rate, viscositycharacteristics have a great contribution, and the surface HMC structureaccording to the present invention exhibits response on high straindeformation in the viscosity characteristics, making it possible toimprove impact resistance. Therefore, the high strength polyethylenefibers of the present invention having such improved impact resistanceare suitable for ballistic materials or items, or as the reinforcingfibers of helmets.

[0031] Thus the high strength polyethylene fibers of the presentinvention have remarkably improved durability, particularly abrasionresistance, as compared with the conventional high strength polyethylenefibers. More specifically, the number of frictions until the fiber isbroken in an abrasion test according to method B for measuring abrasionresistance in the Testing Methods for Spun Yarn (JIS L 1095) is 100,000or larger.

[0032] The high strength polyethylene fibers of the present inventionshould be produced with deliberation by a novel method of production,for example, the method described below, which is recommended, but itis, of course, not limited thereto.

[0033] First of all, a high molecular weight polyethylene as describedabove is uniformly dissolved in a solvent to give a spinning solution.The spinning solution has a concentration of usually 50% or lower,preferably 30% or lower. The solvent may include volatile solvents suchas decalin or tetralin and non-volatile solvents such as paraffin oil orparaffin wax. The use of volatile solvents is preferred. This is becausefor solvents which are in solid state or nonvolatile at ordinarytemperature, the rate of solvent extraction from filaments is slow andit is, therefore, difficult to achieve sufficient formation of HMC,whereas volatile solvents in the fiber surface are positively evaporatedin the spinning to give a higher concentration in the fiber surface,making it possible to form a specific crystal structure (HMC) in whichmolecular chains are more highly oriented and connected with each other.In case of conventional spinning techniques, a structural differencebetween the fiber surface and the inside is responsible for a decreasein the strength of fibers; the selection of spinning conditions to makethe sectional structure of fibers as uniform as possible is, therefore,common knowledge for persons of ordinary skill in the art of not onlygel spinning but also dry spinning, wet spinning, and melt spinning ofpolyvinyl alcohol and polyacrylonitrile, for example, i.e., in the artof spinning in general.

[0034] On the contrary, the present inventors have found that theformation of a structural difference between the fiber surface and theinside at the step of spinning, more specifically the formation of HMCby instant and positive removal of solvents in the fiber surface tothereby concentrate the tension of spinning on the surface layer, makesit possible to obtain fibers keeping high strength and high Young'smodulus and further having excellent flexural fatigue resistance andexcellent abrasion resistance.

[0035] In the production of the high strength polyethylene fibers of thepresent invention, there is recommended a technique of blowing ahigh-temperature inert gas onto the discharged filaments just below thespinneret for the positive removal of solvents on the surface of thefilaments. This results in the formation of a very thin HMC layer on thesurface to thereby concentrate the tension of spinning, making itpossible to form a specific structure in which molecular chains areconnected with each other as described above. The temperature of theinert gas is usually 60° C. or higher, preferably 80° C. or higher, andmore preferably 100° C. or higher but below 150° C. For the inert gas,the use of a nitrogen gas is preferred from an economical point of view,but it is not limited thereto.

[0036] The unstretched filaments thus obtained are heated again toremove the remaining solvents, during which they are drawn at a ratio ofseveral times. Depending on the situation, multi-step drawing may beemployed. The HMC structure of the surface layer formed in the spinningcan never be eliminated in the later drawing steps, making it possibleto obtain high strength polyethylene fibers having extremely excellentcharacteristics as described above. The high strength polyethylenefibers obtained, even if cut, hardly get stuck together by pressure whenthey are cut because of their having a close structure in the surface,although such sticking phenomenon is observed in the conventionalfibers; therefore, chopped fibers or staples having good dispersibilitycan be obtained.

[0037] The high strength polyethylene fibers of the present inventionhave excellent flexural fatigue resistance and excellent abrasionresistance, while having about the same or higher strength and Young'smodulus than those of the conventional high strength polyethylenefibers; therefore, the high strength polyethylene fibers of the presentinvention are suitable for various ropes or cables for industrial orprivate use, particularly running cables used for a long period, such asmooring ropes and hawsers; blind cables; printer cables; and they arealso useful as the materials for various sports gears and sportswears,such as fishing lines, tents, sports socks and uniforms, and variousgarments. The high strength polyethylene fibers of the present inventionare also extremely useful for high performance textiles such asballistic materials or items, or protective gloves because of theirexcellent cut resistance and excellent blade resistance arising from theabove excellent characteristics. The high strength polyethylene fibersof the present invention are further useful in the chemical processesbecause of their having close surface and therefore having remarkablyimproved resistance to chemicals, light and weather, as compared withthe conventional ultrahigh molecular weight polyethylene fibers, forexample, as the chopped fibers to produce non-woven fabrics such aschemical filters or battery cell separators, which are required to havechemical resistance. The high strength polyethylene fibers of thepresent invention are still further useful as the reinforcing fibers inthe composite materials for sports gears such as helmets and skis andfor speaker cones, as the reinforcing materials for concrete or mortar,particularly spray-coated concrete or normal plane concrete in tunnels,or as the fibers for reinforcing sheets and strands to provideearthquake resistance.

[0038] Now, among the applications of the high strength polyethylenefibers of the present invention, the following will describe choppedfibers, ropes, nets, ballistic materials or items, protective gloves,and fiber reinforcing concrete products, in particular.

[0039] The high strength polyethylene chopped fibers of the presentinvention can be obtained from the above novel high strengthpolyethylene fibers, and the amount of poorly dispersed fibers (fiberbundles of 40 μm or larger in maximum diameter formed by getting stucktogether by pressure or fusion) found in case of paper making ispreferably 5% by weight or smaller. If the amount of fiber bundles of 40μm or larger in maximum diameter is greater than 5% by weight, unevensuction may occur to form spots when water is sucked under reducedpressure in the wet process of producing non-woven fabrics. Theformation of spots is responsible for the deterioration of strength,stab resistance, and other characteristics of non-woven fabrics. Themono-filament linear density of chopped fibers is not particularlylimited, but usually in the range of 0.1 to 20 dpf. It may be changedaccording to the applications, for example, those of large lineardensity are used as the reinforcing fibers for concrete or cement, andfor ordinary non-woven fabrics; and those of small linear density areused for high-density non-woven fabrics such as chemical filters orbattery cell separators. The length of chopped fibers, i.e., the cutlength of fibers, is preferably 70 mm or shorter, more preferably 50 mmor shorter. This is because if the cut length is too long, fibers easilycause entanglement to make their uniform dispersion difficult. Further,the method of cutting fibers may include, but is not particularlylimited to, those of the guillotine type or the rotary cut type.

[0040] The high strength polyethylene chopped fibers of the presentinvention are useful as the chopped fibers to produce non-woven fabricssuch as chemical filters, battery cell separators, and water-shieldsheets for chemicals; as the reinforcing fibers for concrete or cement;and as the staples to produce feather blankets or spun yarns, because oftheir excellent resistance to chemicals, light and weather.

[0041] The ropes of the present invention can be produced using theabove novel high strength polyethylene fibers as the base yarns, whichfibers may be mixed with other fibers known in the art. Depending uponthe design or function, for example, the ropes of the present inventionmay be covered on their surface with different materials such as lowmolecular weight polyolefins or urethane resins. The ropes of thepresent invention may be in any form, including twist structures, suchas three-ply ropes and six-ply ropes; braid structures, such aseight-ply ropes, twelve-ply ropes, and double-braided ropes; anddouble-braid structures in which the core part of the ropes is helicallycovered on their periphery with yarns or strands; in any way, they maybe designed for ropes of the ideal type adapted to their applicationsand performance.

[0042] The ropes of the present invention exhibit less deterioration intheir performance by moisture absorption, water absorption, or otherfactors, and even if they are of small diameters, they have highstrength, causes no kink and further have good storage characteristics.For this reason, the ropes of the present invention are the mostsuitable as various ropes for industrial or private use, such as fisheryropes, tag ropes, mooring ropes, hawsers, yacht ropes, climbing ropes,farming ropes, various kinds of ropes for civil engineering,architecture, wiring, and construction works, particularly for watersideapplications related to shipping and fishery.

[0043] The nets of the present invention can be produced using the abovenovel high strength polyethylene fibers as the base yarns, which fibersmay be mixed with other fibers known in the art. Depending upon thedesign or function, for example, the nets of the present invention maybe covered on their periphery with different materials such as lowmolecular weight polyolefins or urethane resins. The nets of the presentinvention may be in any form, including knotted, knotless, Russell, andother structures; in any way, they may be designed for nets of the idealtype adapted to their applications and performance.

[0044] The nets of the present invention have strong mesh, excellentflexural fatigue resistance, and excellent abrasion resistance. For thisreason, the nets of the present invention are useful as various nets forindustrial or private use, such as various fishery nets including trawlnets, stationary nets, round haul nets, gauze nets, and gill nets;various farming nets for. protection against beasts and birds; varioussports nets including golf nets and protection nets against balls;safety nets; and various nets for civil engineering, wiring, andconstruction works.

[0045] The ballistic materials or items of the present invention can beproduced using the above novel high strength polyethylene fibers as thebase yarns, which fibers may be mixed with other fibers known in theart. The ballistic materials or items of the present invention may beproduced by, for example, weaving the base yarns into cloths, orparalleling the base yarns along one direction and then impregnating theparalleled yarns with resins, followed by layering to form sheets sothat the paralleling direction for one sheet is perpendicular to thatfor the next sheet; and laminating such two or more cloths or sheets.

[0046] The protective gloves of the present invention can be producedusing the above novel high strength polyethylene fibers as the baseyarns, which fibers may be mixed with the other fibers known in the art.The protective gloves of the present invention may be provided withfunctionality, including sweat absorption by mixing the base fibers withmoisture-absorptive fibers such as cotton fibers, or improvement in thefeeling of a fit when wore by mixing the base fibers with urethane-typestretch fibers. For design, the gloves of the present invention may becolored by mixing the base yarns with colored yarns, making it difficultto attract attention to their dirty parts or making it possible toimprove their fashionability. The method of mixing the high strengthpolyethylene fiber filaments with other fibers may include interlacingtreatment using air entanglement, or Taslan treatment, and may furtherinclude mixing with other fibers after opening the filaments by theapplication of voltage, or simple twisting, braiding or covering withthe other fibers. When they are used as staples, the filaments are mixedwith other fibers during the production of spun yarns, or after theproduction using the above method of mixing.

[0047] The protective gloves of the present invention have excellent cutresistance against sharp objects such as blades, as compared withprotective gloves made of the conventional high strength polyethylenefibers. This is probably because the high strength polyethylene fibersof the present invention used have high surface hardness because oftheir HMC structure on the fiber surface. Therefore, the protectivegloves of the present invention are useful as the gloves for workingscenes requiring high cut resistance.

[0048] The fiber reinforced concrete products of the present inventioncan be produced using the above novel high strength polyethylene fibersas the reinforcing fibers. The reinforcing fibers have excellent cutresistance, probably because of their close surface, and when dispersedin a cement matrix, they hardly cause the flexure of fibers and exhibitgood dispersion properties in the cement matrix. The reinforcing fibershave further improved resistance to chemicals, light and weather,because of their close surface, as compared with the conventional highstrength polyethylene fibers, and they are the most suitable as thereinforcing fibers, particularly for concrete or cement, which isrequired to have chemical resistance against the alkaline properties ofcement. Therefore, the fiber reinforced concrete products of the presentinvention exhibit good workability in their production, and haveimproved performance such as compressive strength, flexural strength andtoughness, and further have excellent impact resistance and excellentdurability.

EXAMPLES

[0049] The present invention will be further illustrated by someexamples; however, the present invention is not limited to theseexamples.

[0050] First, the high strength polyethylene fibers of the presentinvention are exemplified by Examples 1 to 3 and Comparative Example 1to 2. The polyethylene fibers prepared by these Examples and ComparativeExamples were measured for their physical properties by the followingmeasuring and testing methods, and evaluated for their performance.

[0051] Intrinsic Viscosity Number of Fibers

[0052] Using a capillary viscosity tube of the Ubbeloahde type, dilutesolutions of different concentrations were measured for viscosity indecalin at 135° C., and intrinsic viscosity number was determined bydrawing a straight line on the plot of their specific viscosity againstconcentrations by the method of least squares and extrapolation of thestraight line toward zero concentration. In the measurement ofviscosity, a specimen was cut to about 5 mm in length, and anantioxidant (under the trade name “Yoshinox BHT” available fromYoshitomi Pharmaceutical Industries, Ltd.) was added in 1 wt % relativeto the specimen, followed by stirring at 135° C. for 4 hours fordissolution to give a solution for measurement.

[0053] Strength and Young's Modulus of Fibers

[0054] Using “Tensilon” available from Orientech Corp., a strain-stresscurve was drawn under the conditions: length of specimen, 200 mm (gapdistance between chucks); elongation rate, 100%/min.; atmospherictemperature, 20° C.; and relative humidity, 65%; and strength (cN/dtex)was calculated from the stress at the breaking point on the curveobtained, and Young's modulus (cN/dtex) was calculated from the tangentline providing the maximum gradient on the curve in the vicinity of theorigin. The number of measurements was set at 10, and the strength andYoung's modulus were expressed by the respective averaged values.

[0055] Differential Scanning Calorimetry (DSC) of Fibers

[0056] Using “DSC7” available from Perkin-Elmer Corp. (the maximumsensitivity, 8 μW/cm), DSC was carried out as follows. A specimen wascut in 5 mm or shorter, and about 5 mg of the specimen was charged andsealed in an aluminum pan. The same, but vacant, aluminum pan was usedas a reference. A temperature-increasing DSC curve was drawn by heatingthe specimen from room temperature up to 200° C. at a heating rate of10° C./min., under an atmosphere of an inert gas. Thetemperature-increasing DSC curve obtained was corrected for the baseline, followed by reading the number of peaks, peak temperatures, andpeak heights over a temperature region of 140° C. to 148° C. (on the lowtemperature side) and over a temperature region of 148° C. or higher (onthe high temperature side), and calculating the height ratio of themaximum endothermic peak on the low temperature side and the maximumendothermic peak on the high temperature side. If the endothermic peaksare difficult to read because of their shoulder-like shapes, the valuesof heat flow at 145.5° C. and at 150° C. were regarded as theendothermic peaks on the low temperature side and on the hightemperature side, respectively, to calculate the ratio of peak heights.

[0057] Abrasion Test of Fibers

[0058] A specimen was prepared by multiplying or adjustment to have alinear density of about 1500 dtex and evaluated for abrasion resistanceby an abrasion test according to method B for measuring abrasionresistance in the Testing Methods for Spun Yarns (JIS L 1095). Using a0.9 mmφ tip of hard steel as a friction contact, each test was carriedout under the conditions: load, 0.5 g/d; rate of friction, 115times/min.; distance of reciprocating motion, 2.5 cm; and angle offriction, 110°. The abrasion resistance was determined as the number offrictions until a specimen was broken. The number of runs was set at 2,and the results were expressed by their average values. The values wererounded off to the third digit.

[0059] Example 1

[0060] A slurry mixture of 10 wt % ultrahigh molecular weightpolyethylene having an intrinsic viscosity number of 21.0 and amolecular weight distribution index (“1w/Mn) of 3.7 and 90 wt % decalinwas fed to a screw-type kneader at 230° C., and their dissolution wasachieved to give a spinning solution, followed by spinning at a rate ofdischarge through each nozzle of 1.4 g/min. using a spinneret (thediameter of each nozzle, 0.7 mm; the number of nozzles, 400) at 170° C.The discharged filaments were blown with a nitrogen gas at 100° C. asuniformly as possible at an average flow rate of 1.2 m/sec. through aslit-shaped orifice for gas feed, which orifice was disposed just belowthe spinneret, so that the decalin in the fiber surface was positivelyevaporated. Immediately after that, the filaments were substantiallycooled with an air flow at 30° C., and wound at a rate of 75 m/min. byNelson-type rollers disposed downstream the spinneret. At that time, thesolvent contained in the filaments had already reduced its weight byhalf from the original weight. Subsequently, the filaments obtained weredrawn at a ratio of 4 in a heating oven at 100° C. and further drawn ata ratio of 4 in a heating oven at 149° C., thereby obtaining apolyethylene fiber. The physical properties and performance evaluationof the fiber are shown in Table 1. The temperature-increasing DSC curvebefore base line correction, which was obtained by differential scanningcalorimetry (DSC), is shown in FIG. 1.

Example 2

[0061] A polyethylene fiber was prepared in the same manner as describedin Example 1, except that the discharged filaments were blown with anitrogen gas at 120° C. at an average flow rate of 1.4 m/sec. Thephysical properties and performance evaluation of the fiber are shown inTable 1. The temperature-increasing DSC curve before base linecorrection, which was obtained by differential scanning calorimetry(DSC), is shown in FIG. 2.

Example 3

[0062] A polyethylene fiber was prepared in the same manner as describedin Example 1, except that a high molecular weight polyethylene having anintrinsic viscosity number of 12.1 and a molecular weight distributionindex (Mw/Mn) of 5.4 was used; the concentration of a spinning solutionwas set at 30 wt %; and drawing was carried out at a ratio of 3 for thefirst stage and 2.2 for the second stage. The physical properties andperformance evaluation of the fiber are shown in Table 1. Thetemperature-increasing DSC curve before base line correction, which wasobtained by differential scanning calorimetry (DSC), is shown in FIG. 3.

Comparative Example 1

[0063] A polyethylene fiber was prepared in the same manner as describedin Example 1, except that the discharged filaments were not blown justbelow the spinneret with a high temperature nitrogen gas, but wereimmediately cooled with a nitrogen gas at 30° C.; and drawing wascarried out at a ratio of 4.0 for the first stage and 3.5 for the secondstage. The physical properties and performance evaluation of the fiberare shown in Table 1. The temperature-increasing DSC curve before baseline correction, which was obtained by differential scanning calorimetry(DSC), is shown in FIG. 4.

Comparative Example 2

[0064] Spinning was carried out in the same manner as described inExample 1, except that paraffin oil was used as a solvent; and drawingwas carried out at a ratio of 4, while the solvent was substantiallyextracted in a cooling bath containing n-decane at about 80° C., whichbath was disposed just below the spinneret. No positive cooling with aninert gas was carried out. The semi-stretched filaments obtained werefurther drawn at a ratio of 4 in an oven at 145° C. under an atmosphereof an inert gas, so that the n-decane contained was substantiallyevaporated, thereby obtaining a polyethylene fiber. The physicalproperties and performance evaluation of the fiber are shown in Table 1.The temperature-increasing DSC curve before base line correction, whichwas obtained by differential scanning calorimetry (DSC), is shown inFIG. 5. TABLE 1 Number at maximum Mono- Number of Number of endothermicpeak Intrinsic filament frictions endothermic peaks (° C.) Height ratioviscos- Linear linear Young's until fibers Low High Low High of maximumity density density Strength modulus are broken temperature temperaturetemperature temperature endothermic number (dtex) (dtex) (cN/dtex)(cN/dtex) (times) side side side side peaks Example 1 18.5 455 1.2 38.11521 356,000 1 2 142.0 148.5 2.4:1 Example 2 18.4 448 1.2 35.2 1612421,000 1 1 144.7 151.3 2.4:1 Example 3  9.4 1150  1.2 28.5 1055 381,0001 1 144.3 151.7 2.0:1 Comp. Ex. 1 18.4 541 1.2 34.2 1516  98,000 0 1145.5 —  (3.2:1)* Comp. Ex. 2 18.3 471 1.2 35.7 1623  57,000 0 1 145.5 ——

[0065] As can be seen from Table 1, the polyethylene fibers of Examples1 to 3 had extremely excellent abrasion resistance because of their 3.5times larger numbers of frictions until the fibers were broken in theabrasion test, while exhibiting about the same or higher strength andYoung's modulus, as compared with the polyethylene fibers of ComparativeExamples 1 to 2. For the polyethylene fibers of Examples 1 to 3, thetemperature-increasing DSC curves had one or two endothermic peaks onthe high temperature side and one endothermic peak on the lowtemperature side. In contrast, for the polyethylene fiber of ComparativeExample 1, the temperature-increasing DSC curve had no endothermic peakon the low temperature side, whereas no distinct peak but a shoulder wasfound as the endothermic peak on the high temperature side. For thepolyethylene fiber of Comparative Example 2, the temperature-increasingDSC curve had one complete peak on the high temperature side, whereas noendothermic peak but a small shoulder around 133° C. was observed on thelow temperature side. For the polyethylene fibers of ComparativeExamples 1 and 2, it is presumed that their maximum endothermic peaks onthe high temperature side come from EC and not from HMC, because oftheir extremely deteriorated abrasion resistance.

[0066] Now, the high strength polyethylene chopped fibers of the presentinvention are exemplified by Examples 4 to 8 and Comparative Examples 3to 5. The chopped fibers prepared in these Examples and ComparativeExamples were evaluated for their performance by the following testingmethod.

[0067] Dispersibility Test of Chopped fibers

[0068] First, 0.02 g of chopped fibers was weighed, put into a beakercontaining 300 ml of distilled water, stirred 50 times with a glass rod.The chopped fibers were then collected by filtering with a fine net suchthat they could not pass through, followed by air drying for 24 hours.Then, fiber bundles formed by getting stuck together by pressure orfusion were taken out by observation with a loupe. The fiber bundleswere measured for their diameter with a microscope, and the fiberbundles of 40 μm or larger in maximum diameter (poorly dispersed fibers)were measured for their total weight. Further, the weight including thatof chopped fibers having good dispersibility was measured, and thecontent of poorly dispersed fibers (percent dispersion failure) wascalculated. In this test, the results seems to vary widely; therefore,the number of runs was set at 10, and the results were expressed bytheir average values.

Examples 4

[0069] The high strength polyethylene fibers prepared in Example 1(intrinsic viscosity number, 18.5; linear density, 455 dtex; strength,38.1 cN/-dtec; Young's modulus, 1521 cN/dtex) were cut in 10 mm by theguillotine method to give chopped fibers. Their performance evaluationis shown in Table 2.

Examples 5

[0070] The high strength polyethylene fibers prepared in Example 2(intrinsic viscosity number, 18.4; linear density, 448 dtex; strength,35.2 cN/-dtec; Young's modulus, 1612 cN/dtex) were cut in 10 mm by theguillotine method to give chopped fibers. Their performance evaluationis shown in Table 2.

Examples 6

[0071] The high strength polyethylene fibers prepared in Example 3(intrinsic viscosity number, 9.4; linear density, 1150 dtex; strength,28.5 cN/-dtec; Young's modulus, 1055 cN/dtex) were cut in 10 mm by theguillotine method to give chopped fibers. Their performance evaluationis shown in Table 2.

Example 7

[0072] A polyethylene fiber was prepared in the same manner as describedin Example 1, except that a spinneret (the diameter of each nozzle, 0.2mm; the number of nozzles, 200) was used and the rate of dischargethrough each nozzle was set at 0.08 g/min. For the fiber obtained, theintrinsic viscosity number was 18.5, the linear density was 240 dtex,the monofilament linear density was 0.12 dtex, the strength was 33.6cN/dtex, the Young's modulus was 1342 cN/dtex, the number of frictionsuntil the fiber was broken in the abrasion test was 103,000, and thetemperature-increasing DSC curve obtained by differential scanningcalorimetry (DSC) had one endothermic peak on the low temperature sideand two endothermic peaks on the high temperature side, with thetemperature of the maximum endothermic peak being 144.7° C. on the lowtemperature side and 159.2° C. on the high temperature side, and theheight ratio of the maximum endothermic peaks being 2.4:1. This fiberwas cut in 50 mm by the guillotine method to give chipped fibers. Theirperformance evaluation is shown in Table 2.

Example 8

[0073] A polyethylene fiber was prepared in the same manner as describedin Example 1, except that a high molecular weight polyethylene having anintrinsic viscosity number of 10 and a molecular weight distributionindex (Mw/Mn) of 5.4 was used; the concentration of a spinning solutionwas set at 30 wt %; a spinneret (the diameter of each nozzle, 0.2 mm;the number of nozzles, 200) was used; and the rate of discharge througheach nozzle was set at 0.08 g/min. For the fiber obtained, the intrinsicviscosity number was 9.4, the linear density was 1265 dtex, themonofilament linear density was 0.63 dtex, the strength was 25.2cN/dtex, the Young's modulus was 931 cN/dtex, the number of frictionsuntil the fiber was broken in the abrasion test was 161,000, and thetemperature-increasing DSC curve obtained by differential scanningcalorimetry (DSC) had one endothermic peak on the low temperature sideand two endothermic peaks on the high temperature side, with thetemperature of the maximum endothermic peak being 143.9° C. on the lowtemperature side and 154.9° C. on the high temperature side, and theheight ratio of the maximum endothermic peaks being 2.2:1. This fiberwas cut in 10 mm by the guillotine method to give chopped fibers. Theirperformance evaluation is shown in Table 2.

Comparative Examples 3

[0074] The high strength polyethylene fibers prepared in ComparativeExample 1 (intrinsic viscosity number, 18.4; linear density, 541 dtex;strength, 34.2 cN/dtec; Young's modulus, 1516 cN/dtex) were cut in 10 mmby the guillotine method to give chipped fibers. Their performanceevaluation is shown in Table 2.

Comparative Examples 4

[0075] The high strength polyethylene fibers prepared in ComparativeExample 2 (intrinsic viscosity number, 18.3; linear density, 471 dtex;strength, 35.7 cN/dtec; Young's modulus, 1623 cN/dtex) were cut in 10 mmby the guillotine method to give chopped fibers. Their performanceevaluation is shown in Table 2.

Comparative Example 5

[0076] A polyethylene fiber was prepared in the same manner as describedin Example 1, except that paraffin oil was used as a solvent; and thefilament was drawn at a ratio of 4 in a heating oven at 100° C. and thenat a ratio of 4 in a heating oven at 149° C. For the fiber obtained, theintrinsic viscosity number was 18.5, the linear density was 455 dtex,the monofilament linear density was 1.2 dtex, the strength was 38.1cN/dtex, the Young's modulus was 1521 cN/dtex, the number of frictionsuntil the fiber was broken in the abrasion test was 421,000, and thetemperature-increasing DSC curve obtained by differential scanningcalorimetry (DSC) had one endothermic peak on the low temperature sideand two endothermic peaks on the high temperature side, with thetemperature of the maximum endothermic peak being 144.3° C. on the lowtemperature side and 152.1° C. on the high temperature side, and theheight ratio of the maximum endothermic peaks being 2.4:1. This fiberwas cut in 80 mm by the guillotine method to give chopped fibers. Theirperformance evaluation is shown in Table 2. TABLE 2 Intrinsic LinearMonofilament Cut Dispersion viscosity density linear density lengthfailure number (dtex) (dtex) (mm) (%) Example 4 18.5 455 1.2 10 2.4Example 5 18.4 448 1.2 10 2.6 Example 6 9.4 1150 1.2 10 1.8 Example 718.5 240 0.12 50 3.9 Example 8 9.4 1265 0.63 10 3.9 Comp. Ex. 3 18.4 5411.2 10 11.7 Comp. Ex. 4 18.3 471 1.2 10 15.1 Comp. Ex. 5 18.5 455 1.2 807.1

[0077] As can be seen from Table 2, the chopped fibers of Examples 4 to8 exhibited low dispersion failure and therefore had excellentdispersibility, as compared with the chopped fibers of ComparativeExamples 3 to 5.

[0078] Now, the ropes using the high strength polyethylene fibers of thepresent invention are exemplified by Examples 9 to 10 and ComparativeExamples 6 to 7. The ropes prepared in these Examples and ComparativeExamples were evaluated for their performance by the following methods.

[0079] Strength Measurement and Flexural Fatigue Test of Ropes

[0080] A rope was fixed on both ends with resin sockets (“Socket Strong”available from Sugita Industry, Co., Ltd.), and used as the testspecimen. The rope was measured for strength at a draw speed of 20cm/min. using “Servo Pulser™” available from Shimadzu Corporation. Therope was immersed in water at room temperature for 24 hours, and thenimmediately measured for strength by the same method, therebydetermining the wet strength of the rope. Further, the flexural fatiguetest was carried out by running the rope through a pulley of 250 mmφ andrepeatedly flexing 5×10⁵ times under a load corresponding to 20% ofbreaking strength. After the test, the rope was fixed again with resinsockets, and then measured for strength to calculate the residualstrength (%).

Example 9

[0081] The high strength polyethylene fiber prepared in Example 1(intrinsic viscosity number, 18.5; linear density, 455 dtex; strength,38.1 cN/dtec; Young's modulus, 1521 cN/dtex) was multiplied foradjustment of linear density, followed by twisting at 100 times/m togive a base yarn. This base yarn was used to prepare a six-ply rope(with a wire rope structure) of about 10 mmφ in thickness. Theperformance evaluation is shown in Table 3.

Example 10

[0082] A rope was prepared in the same manner as described in Example 9,except that the high polyethylene fiber prepared in Example 2 (intrinsicviscosity number, 18.4; linear density, 448 dtex; strength, 35.2cN/dtec; Young's modulus, 1612 cN/dtex) was used. The performanceevaluation is shown in Table 3.

Comparative Example 6

[0083] A rope was prepared in the same manner as described in Example10, except that a commercially available nylon fiber (linear density,467 dtex; strength, 7.3 cN/dtec; Young's modulus, 44 cN/dtex) was used.The performance evaluation is shown in Table 3.

[0084] Comparative Example 7

[0085] A rope was prepared in the same manner as described in Example10, except that a commercially available polyethylene terephthalatefiber (linear density, 444 dtex; strength, 7.4 cN/dtec; Young's modulus,106 cN/dtex) was used. The performance evaluation is shown in Table 3.TABLE 3 Residual strength Rope Wet after 5 × 10⁵ times diameter Strengthstrength repeated flexing (mm) (10⁶ g) (10⁶ g) (%) Example 9 10 3.8 3.8100 Example 10 10 3.5 3.5 100 Comp. Ex. 6 10 2.0 1.8  90 Comp. Ex. 7 102.3 2.3  75

[0086] As can be seen from Table 3, the ropes of Examples 9 and 10exhibited higher strength and higher wet strength, and there wasobserved no decrease in the strength after repeated flexing; therefore,they had excellent flexural fatigue resistance, as compared with theropes of Comparative Examples 6 and 7.

[0087] Now, the nets using the high strength polyethylene fibers of thepresent invention are exemplified by Examples 11 to 12 and ComparativeExamples 8 and 9. The nets prepared in these Examples and ComparativeExamples were evaluated for their performance by the following testingmethods.

[0088] Strength Measurement. Flexural Fatigue Test, and Abrasion Test ofNets

[0089] The measurement of strength was carried out in dry state using“Servo Pulser™” available from Shimadzu Corporation under theconditions: gap distance between chucks, 20 cm; and draw rate, 10cm/min. according to the measuring method for strength in the TestingMethods for Synthetic Fiber Fishing Net Meshes (JIS L 1043). Further,the flexural fatigue test was carried out by fixing the net at both endswith resin sockets (“Socket Strong” available from Sugita Industry Co.,Ltd.), running the net through a pulley of 250 mm+, and repeatedlyflexing 5×10⁵ times under a load corresponding to 20% of breakingstrength. After the test, the socket parts were removed from the net bycutting, and the net was measured for strength in the same manner asdescribed above to calculate residual strength (%). The abrasion testwas carried out by measuring the number of frictions until the twistedyarn was broken by friction in water against a Tungalloy™ edge as afriction contact at an angle of friction of 120° using a fishing nettesting machine under the conditions: distance of frictional motion, 12cm; rate of friction, 30 times/min.; and drawing load, 1 kg.

Example 11

[0090] The high strength polyethylene fiber prepared in Example 1(intrinsic viscosity number, 18.5; linear density, 455 dtex; strength,38.1 cN/dtec; Young's modulus, 1521 cN/dtex) was adjusted to 1200 d,followed by twisting at 180 times/m to give a base yarn. Four base yarnsof this type were twisted at 120 times/m to give a net. The performanceevaluation is shown in Table 4.

Example 12

[0091] A net was prepared in the same manner as described in Example 11,except that the high polyethylene fiber prepared in Example 2 (intrinsicviscosity number, 18.4; linear density, 448 dtex; strength, 35.2cN/dtec; Young's modulus, 1612 cN/dtex) was used. The performanceevaluation is shown in Table 4.

Comparative Example 8

[0092] A net was prepared in the same manner as described in Example 11,except that the high strength polyethylene fiber prepared in ComparativeExample 1 (intrinsic viscosity number, 18.4; linear density, 541 dtex;strength, 34.2 cN/dtec; Young's modulus, 1516 cN/dtex) was used. Theperformance evaluation is shown in Table 4.

Comparative Example 9

[0093] A net was prepared in the same manner as described in Example 11,except that the high strength polyethylene fiber prepared in ComparativeExample 2 (intrinsic viscosity number, 18.3; linear density, 471 dtex;strength, 35.7 cN/dtec; Young's modulus, 1623 cN/dtex) was used. Theperformance evaluation is shown in Table 4. TABLE 4 Residual strengthNumber of after 5 × 10⁵ frictions until Strength times repeated breakingConstitution (N) flexing (times) Example 11 1200 d × 4 1137 100 2260Example 12 1200 d × 4 1176 100 2340 Comp. Ex. 8 1200 d × 4 1052  90  988Comp. Ex. 9 1200 d × 4 1025  87  875

[0094] As can be seen from Table 4, the nets of Examples 11 and 12exhibited higher strength, no decrease was observed in the strengthafter repeated flexing, and the number of frictions until breaking wasat least two times larger; therefore, they had excellent flexuralfatigue resistance and excellent abrasion resistance, as compared withthe nets of Comparative Examples 8 and 9.

[0095] Now, the fiber reinforced concrete products using the highstrength polyethylene fibers of the present invention are exemplified byExamples 13 to 15 and Comparative Examples 10 to 12. The concrete testpieces prepared in these Examples and Comparative Examples wereevaluated for their performance by the following test for strength.

[0096] Test for Strength of Concrete Test Pieces

[0097] In the compressive test, the maximum load was measured todetermine the compressive strength. In the flexural test, the maximumload was measured to determine the flexural strength according to themethod for measuring flexural strength in the Physical Testing Methodsfor Cement (JIS R 5201). For toughness, the relationship between theload and the cross-head displacement in the testing machine was recordedonto an X-Y recorder (available from Yokogawa Electric Corporation), andthe area under the flexural stress-deflection curve until thedisplacement falling down to 50% of the value at the maximum load wasdetermined, and the toughness was regarded as an area ratio with thearea in case of no fiber incorporation being taken as 1.

Example 13

[0098] The high strength polyethylene fiber prepared in Example 1(intrinsic viscosity number, 18.5; linear density, 455 dtex; strength,38.1 cN/dtex; Young's modulus, 1521 cN/dtex) was cut in 30 mm length andused as the reinforcing fibers. First, high-early-strength portlandcement (specific gravity, 3.13), Toyoura-keisa-sand (old standard sand;specific gravity, 2.7) as fine aggregate, and silica fume (specificgravity, 2.2) as mineral admixture were placed in a 5 L volumeomni-mixer and dry mixed for 15 seconds. The above reinforcing fiberswere then put into the mixer, followed by dry mixing for another 30seconds. Water and air entraining and high-range water reducing agent(without incorporation of auxiliary air entraining agents) were then putinto the mixer, followed by mixing for 4 minutes to give fiberreinforced concrete. The water-to-binder ratio was set at 33%; themixing ratio of silica fume relative to the weight of cement, 10%; thesand-to-binder ratio, 60%; the mixing ratio of air entraining andhigh-range water reducing agent relative to the weight of binder, 2.0%;and the mixing ratio of fiber by volume, 2.0%. The measured flow valuesare shown in Table 5.

[0099] Using the fiber reinforced concrete obtained, three cylindricaltest pieces (50 mmφ×100 mm) for compressive test and three prismatictest pieces (40×40×160 mm) for flexural test were prepared byhand-placing with a mallet and a trowel. The test pieces thus obtainedwere subject to the standard cure over 14 days before the test forstrength. The results are shown in Table 5.

Example 14

[0100] A fiber reinforced concrete test piece was prepared in the samemanner as described in Example 13, except that the high polyethylenefiber prepared in Example 2 (intrinsic viscosity number, 18.4; lineardensity, 448 dtex; strength, 35.2 cN/dtec; Young's modulus, 1612cN/dtex) was used; and then subjected to the test for strength. Theresults are shown in Table 5.

Example 15

[0101] A fiber reinforced concrete test piece was prepared in the samemanner as described in Example 13, except that the high polyethylenefiber prepared in Example 3 (intrinsic viscosity number, 9.4; lineardensity, 1150 dtex; strength, 28.5 cN/dtec; Young's modulus, 1055cN/dtex) was used; and then subjected to the test for strength. Theresults are shown in Table 5.

Comparative Example 10

[0102] A fiber reinforced concrete test piece was prepared in the samemanner as described in Example 13, except that the high polyethylenefiber prepared in Comparative Example 1 (intrinsic viscosity number,18.4; linear density, 541 dtex; strength, 34.2 cN/dtec; Young's modulus,1516 cN/dtex) was used; and then subjected to the test for strength. Theresults are shown in Table 5.

Comparative Example 11

[0103] A fiber reinforced concrete test piece was prepared in the samemanner as described in Example 13, except that the high polyethylenefiber prepared in Comparative Example 2 (intrinsic viscosity number,18.3; linear density, 471 dtex; strength, 35.7 cN/dtec; Young's modulus,1623 cN/dtex) was used; and then subjected to the test for strength. Theresults are shown in Table 5.

Comparative Example 12

[0104] A fiber reinforced concrete test piece was prepared in the samemanner as described in Example 13, except that no reinforcing fiber wasused; and then subjected to the test for strength. The results are shownin Table 5. TABLE 5 Mixing ratio of Compres- fiber by Flow sive Flexuralstrength volume value strength (MPa) Tough- (%) (mm) (MPa) Cracking Max.ness Example 13 2 181 83.8 12.8 22.0 83.6 Example 14 2 172 81.5 12.520.5 78.1 Example 15 2 179 83.2 12.7 21.6 80.5 Comp. Ex. 10 2 150 74.511.8 14.2 24.8 Comp. Ex. 11 2 143 75.5 12.4 13.4 14.3 Comp. Ex. 12 0 15371.1 12.0 12.0 1.0

[0105] As can be seen from Table 5, the test pieces of Examples 13 to 15exhibited higher compressive strength, higher flexural strength, andhigher toughness; therefore, they had excellent impact resistance andexcellent durability, as compared with the test pieces of ComparativeExamples 10 to 12.

[0106] Now, the protective glove materials using the high strengthpolyethylene fibers of the present invention are exemplified by Examples16 to 18 and Comparative Examples 13 to 14. The protective glovematerials prepared in these Examples and Comparative Examples wereevaluated for their performance by the following cutting test.

[0107] Cutting Test for Protective Glove Materials

[0108] The cutting test was carried out using a Coup testa™ (availablefrom SODEMAT Corp.; France). This machine is fabricated such that aspecimen is cut by allowing a round blade to move on the specimen, whichround blade is rotated in the direction opposite to the direction ofmoving, and after the completion of cutting, aluminum foil laid underthe specimen comes in contact with the round blade to turn on theelectricity, thereby sensing the completion of the test. While the roundblade is in operation, a counter provided in the machine shows anumerical value connected with the number of rotation of the roundblade, and this numerical value is recorded.

[0109] In this test, a plain-woven cotton fabric of about 200 g/m² inweight was used as a blank. The cutting level of a specimen relative tothis blank was regarded as the cut resistance, and the evaluation wascarried out as follows. First, the test was started with the blank, andrepeated alternately for the blank and the specimen. After the specimenwas tested five times, the last run was carried out for the blank, and aseries of runs was completed for one specimen. From the counts in therespective runs, the index values were calculated by the followingequation, and the cut resistance was evaluated by their average value inthe test repeated five times.

[0110] Index=(count of sample+A)/A

[0111] A=(count of cotton fabric before test of sample+count of cottonfabric after test of sample)/2

[0112] The round blade used in the test was a 45 mmφ new blade(material, SKS-7 tungsten steel; blade thickness, 0.3 mm) for L-typerotary cutters available from OLFA Corp., and the load applied to aspecimen in the test was set at 320 g.

Examples 16

[0113] Using the high strength polyethylene fiber prepared in Example 1(intrinsic viscosity number, 18.5; linear density, 455 dtex; strength,38.1 cN/dtex; Young's modulus, 1521 cN/dtex), a plain knitted fabric wasprepared with a circular knitting machine. The protective glove materialobtained was tested on the back side with a Coup testa for theevaluation of cut resistance. The results are shown in Table 6.

Example 17

[0114] A plain knitted fabric was prepared in the same manner asdescribed in Example 16, except that the high strength polyethylenefiber prepared in Example 2 (intrinsic viscosity number, 18.4; lineardensity, 448 dtex; strength, 35.2 cN/dtex; Young's modulus, 1612cN/dtex) was used; and then subjected to the cutting test. The resultsare shown in Table 6.

Example 18

[0115] A plain knitted fabric was prepared in the same manner asdescribed in Example 16, except that the high strength polyethylenefiber prepared in Example 3 (intrinsic viscosity number, 9.4; lineardensity, 1150 dtex; strength, 28.5 cN/dtex; Young's modulus, 1055cN/dtex) was used; and then subjected to the cutting test. The resultsare shown in Table 6.

Comparative Example 13

[0116] A plain knitted fabric was prepared in the same manner asdescribed in Example 16, except that the high strength polyethylenefiber prepared in Comparative Example 1 (intrinsic viscosity number,18.4; linear density, 541 dtex; strength, 34.2 cN/dtex; Young's modulus,1516 cN/dtex) was used; and then subjected to the cutting test. Theresults are shown in Table 6.

Comparative Example 14

[0117] A plain knitted fabric was prepared in the same manner asdescribed in Example 16, except that the high strength polyethylenefiber prepared in Comparative Example 2 (intrinsic viscosity number,18.3; linear density, 471 dtex; strength, 35.7 cN/dtex; Young's modulus,1623 cN/dtex) was used; and then subjected to the cutting test. Theresults are shown in Table 6. TABLE 6 Index values 1st run 2nd run 3rdrun 4th run 5th run average Example 16 6.01 6.33 7.26 7.27 6.59 6.69Example 17 5.88 7.20 6.93 6.32 6.80 6.63 Example 18 7.98 7.12 8.12 7.548.66 7.88 Comp. Ex. 13 4.67 4.67 4.40 5.00 6.67 5.08 Comp. Ex. 14 5.454.39 4.82 5.11 5.75 5.10

[0118] As can be seen from Table 6, the protective glove materials ofExamples 16 to 18 exhibited higher index values by 1.0 or more;therefore, they had excellent cut resistance, as compared with theprotective glove materials of Comparative Examples 13 and 14.

[0119] Now, the ballistic materials or items using the high strengthpolyethylene fibers of the present invention are exemplified in Example19 and Comparative Example 15. The ballistic materials prepared in theseExamples and Comparative Example were evaluated for their performance bythe following ballistic test.

[0120] Ballistic Test for Ballistic Materials

[0121] A specimen of materials was cut into squares each measuring 20 cmin one side, and these squares were layered to have a weight of about 1kg/m² and then peripherally fastened with over-locks to give a specimenfor ballistic test.

[0122] The ballistic test was carried out according to MIL-STD-662E. A17-grain fragment simulating projectile as described in MIL-P-46593A wasdischarged at a speed of about 500 m/sec. from a barrel with apropellant to collide with the specimen. At that time, speeds V₁ and V₂of the fragment simulating projectile before the collision with thespecimen and after the penetration, respectively, were measured with achronograph using a photoscreen. The impact resistance was evaluated byunit energy loss (SEA) calculated from the measured speeds V₁ and V₂ bythe following equation.

SEA=½×(V ₁ ² −V ₂ ²)×0.0011/sample weight

[0123] The number of runs was set at 3, and the value of SEA wasexpressed by their average value.

Example 19

[0124] The high strength polyethylene fiber obtained in Example 2(intrinsic viscosity number, 18.4; linear density, 448 dtex; strength,35.2 cN/dtex; Young's modulus, 1612 cN/dtex) was twisted to have a twistcoefficient of 0.7, thereby obtaining a base yarn. A plain-woven fabric(weight, 180.6 g/m²) with the numbers of warp yarns and weft yarns beingboth 48 yarns/25 mm was prepared with this base yarn. The ballisticmaterial obtained was subjected to the ballistic test. The results areshown in Table 7.

Comparative Example 15

[0125] A plain-woven fabric (weight, 188.1 g/m²) was prepared in thesame manner as described in Example 19, except that the high strengthpolyethylene fiber obtained in Comparative Example 2 (intrinsicviscosity number, 18.3; linear density, 471 dtex; strength, 35.7cN/dtex; Young's modulus, 1623 cN/dtex) was used; and then subjected tothe ballistic test. The results are shown in Table 7. TABLE 7 FabricNumber of Weight of weight layers specimen SEA (g/m²) (pieces) (g/m²)(J/kg/m²)) Example 19 180.6 6 1085 25.2 Comp. Ex. 15 188.1 6 1130 23.9

[0126] As can be seen from Table 7, the ballistic material of Example 19exhibited higher unit energy loss; thereby it had excellent impactresistance, as compared with the ballistic material of ComparativeExample 15.

INDUSTRIAL APPLICABILITY

[0127] According to the present invention, high strength polyethylenefibers can be obtained, which have about the same or higher strength andYoung's modulus than those of the conventional high strengthpolyethylene fibers, and at the same time, have excellent durability,particularly flexural fatigue resistance and excellent abrasionresistance. Such high strength polyethylene fibers can be widely appliedin various fields for industry, for example, as the chopped fibers orstaples to produce non-woven fabrics or spun yarns; as various ropes ornets for industrial or private use; as the materials for highperformance textiles such as ballistic materials or items and protectivegloves; or as the reinforcing fibers for composite materials such asfiber reinforced concrete products and helmets.

1. A high strength polyethylene fiber characterized in that: the fiber comprises a high molecular weight polyethylene consisting essentially of a repeating unit of ethylene; it has an intrinsic viscosity number of 5 or larger and an average strength of 22 cN/dtex or higher; and the measurement of the fiber by differential scanning calorimetry (DSC) exhibits a temperature-increasing DCS curve having at least one endothermic peak over a temperature region of 140° C. to 148° C. (on the low temperature side) and at least one endothermic peak over a temperature region of 148° C. or higher (on the high temperature side).
 2. The high strength polyethylene fiber according to claim 1, wherein the height ratio of the maximum endothermic peak on the low temperature side and the maximum endothermic peak on the high temperature side is 1.4:1.0 to 3.0:1.0.
 3. The high strength polyethylene fiber according to claim 2, wherein the height ratio of the maximum endothermic peak on the low temperature side and the maximum endothermic peak on the high temperature side is 1.5:1.0 to 2.9:1.0.
 4. A high strength polyethylene fiber characterized in that: the fiber comprises a high molecular weight polyethylene consisting essentially of a repeating unit of ethylene; it has an intrinsic viscosity number of 5 or larger and an average strength of 22 cN/dtex or higher; and the number of frictions until the fiber is broken in an abrasion test according to method B for measuring abrasion resistance in Testing Methods for Spun Yarn (JIS L 1095) is 100,000 or larger.
 5. A high strength polyethylene chopped fiber obtained from the high strength polyethylene fiber according to claim 1 or
 4. 6. The high strength polyethylene chopped fiber according to claim 5, wherein the amount of poorly dispersed fibers (fiber bundles of 40 μm or larger in maximum diameter formed by getting stuck together by pressure or fusion) found in case of paper making is 5% by weight or smaller.
 7. The high strength polyethylene chopped fiber according to claim 5, wherein the fiber has a cut length of 70 mm or shorter.
 8. A rope comprising the high strength polyethylene fiber according to claim 1 or
 4. 9. A net comprising the high strength polyethylene fiber according to claim 1 or
 4. 10. A ballistic material comprising the high strength polyethylene fiber according to claim 1 or
 4. 11. A protective glove comprising the high strength polyethylene fiber according to claim 1 or
 4. 12. A fiber reinforced concrete product comprising the high strength polyethylene fiber according to claim 1 or
 4. 13. A helmet comprising the high strength polyethylene fiber according to claim 1 or
 4. 